Dense wavelength beam combining (DWBC) is a technique for producing a single, high-brightness, multi-spectral output beam from a plurality of narrow spectral bandwidth input beams. DWBC techniques, which have also sometimes been referred to as dense wavelength multiplexing (DWM) techniques in some prior art, enable multiple relatively low-power single wavelength input beams to be superimposed to produce a single, high-power, high-brightness output beam. DWBC techniques enable output beam power to be scaled directly with the sum of the power produced by the plurality of input beams and produce output beams of quality commensurable with the quality of the individual input beams.
In DWBC systems, a plurality of narrow spectral bandwidth, or single wavelength, input laser beams are emitted from a laser source that comprises a plurality of individual emitters. The multi-spectral output beam is formed by combining, or spatially and directionally overlapping, the plurality of individual input beams with a beam combining element. Beam combining can be achieved by selecting, for each individual input beam, a wavelength and angle of incidence with respect to the beam combining element such that all of the input beams emerge from an overlap region of the beam combining element with a common direction of propagation. All combinations of wavelength and angle of incidence that will yield such a combined beam define a set of allowed wavelength-angle pairs for the system.
In order to produce a single multi-spectral combined output beam from the plurality of laser beams emitted by the laser source, a wavelength-angle pair from the set of allowed wavelength-angle pairs must be selected for each emitter in the laser source. Angle of incidence selection can be accomplished by fixing the relative position of the laser source and beam combining element and placing a position-to-angle transformation lens at a fixed position in the optical path between the laser source and the beam combining element. The position-to-angle transformation lens selects an angle of incidence for each emitter in the laser source by mapping the spatial position of each emitter to a particular angle of incidence with respect to the beam combining element.
For each individual emitter, wavelength selection can be accomplished by providing feedback to the emitter in the form of electromagnetic radiation having the desired wavelength for the emitter. Providing such electromagnetic radiation to the emitter will excite a resonant mode of the emitter that corresponds to the desired output. Thus, providing feedback to the emitter will stimulate the emission of additional electromagnetic radiation having a wavelength equivalent to the wavelength of the feedback. The resonant feedback will thereby narrow the spectral bandwidth of the laser beam emitted by the emitter and center the wavelength spectrum of the emitted beam about the wavelength at which the spectrum of the resonant feedback is centered. This process of providing feedback to an emitter can be referred to as beam wavelength stabilization, or wavelength locking.
Locking the wavelength of each laser beam maps a single wavelength to each emitter in the laser source and creates a set of fixed wavelength-position pairs for the laser source. The position-to-angle transformation lens maps the wavelength-position pair for each emitter in the laser source to a particular wavelength-angle pair. Selecting appropriate wavelength-position pairs ensures that the beam combining element will produce a spatially and directionally overlapped beam. However, if any other wavelengths simultaneously oscillate within the resonant feedback cavity (and are thus coupled into the emitters), the emitters will produce additional parasitic wavelength-position pairs which will not be directionally overlapped by the beam combining element. One downstream consequence of the production of additional parasitic wavelengths is a deterioration of the beam quality in the wavelength combining direction. Furthermore, such parasitic wavelengths can induce temporal fluctuation in the output power by means of modal competition within the laser gain medium.